The present invention relates to a method of regenerating gas-turbine stator vane (nozzle) of which constituting material had been deteriorated or damaged according to the preamble of claim 1 (see, for example,
High temperature parts constituting a gas turbine, particularly a stator vane provided at a first stage corresponding to an inlet of combustion gas for the gas turbine is exposed and contacted to the combustion gas having the highest temperature. Therefore, many cracks are liable to be unavoidably caused to the stator vane. The cracks are mainly caused by thermal fatigue proceeded from thermal stress change occurred at the start and stop operations for the gas turbine. Further, the gas-turbine stator vane suffers various damages such as creep cracking, fatigue cracking, thermal deterioration, oxidation, corrosion, erosion caused by being exposed to a high temperature atmosphere, or FOD caused by the collision with flying objects. In reply to this situation, a Co-based alloy excellent in both heat resistance and repairing property has been used as a material constituting the first stage stator vane of the gas turbine for many years.
Conventionally, when aforementioned damages such as crack, oxidation, erosion, wear or the like are caused, the damaged stator vane of the gas turbine is regenerated in accordance with a method comprising the steps of: grinding or chipping (gouging) an entire cracks formed at the damaged portion of the stator vane in the gas turbine; and conducting TIG (tungsten-inert gas) welding or the like to the ground portion thereby to form a weld-repaired stator vane. Then, the resultant weld-repaired stator vane is successively used for the gas turbine.
However, as an operating temperature is heightened for the purpose of improving a turbine-efficiency, the repairing amount for the damaged portion is greatly increased. In this situation, heat affection by the welding operation during the repairing work becomes remarkable and residual stress in the stator vane is greatly increased, so that it becomes difficult to eliminate the heat affection and remove the residual stress. Moreover, an amount of deformation is also largely increased, and the deformation amount cannot be easily corrected through a standard deformation-correcting work to be performed after the repairing work.
Under these circumstances, there has been conventionally proposed a repairing method by diffusion brazing, which has been disclosed, for example, in
Further, as a method of improving mechanical properties by utilizing HIP (hot isostatic pressing) treatment, for example, Japanese Patent Application (Laid-Open) No.
Furthermore, for example, Japanese Patent Application (Laid-Open) No.
Still further, for example, Japanese Patent Application (Laid-Open) No. SHO. 57-62884 has also proposed a welding treatment technique in which welding defects caused during welding treatment for an abutting portion of a wheel and a rotor blade implanted in the wheel are eliminated by HIP (hot isostatic pressing) treatment.
However, since the cracks caused by the thermal stress or the thermal fatigue resulting from the start and stop operations of the gas turbine are unavoidably generated in a large number, when the conventional repairing method in which the cracked portion is entirely ground and then the ground portion is repair-welded by TIG method is adopted, a repairing amount necessary for the welding is greatly increased. In addition, the heat affection by the welding operation during the repairing work becomes remarkable and residual stress is greatly increased, so that it becomes difficult to eliminate the heat affection and remove the residual stress. Moreover, an amount of deformation is also largely increased, and the deformation amount cannot be easily corrected through a standard deformation-correcting work to be performed after the repairing work.
Further, as explained above, according to the conventional repairing method comprising the steps of: mixing a material having substantially the same composition as that of the base material with the brazing material containing the melting point lowering agent such as Si, B or the like at a predetermined mixing ratio thereby to prepare a mixed material; filling the mixed material into a cracked portion or disposing the mixed material on a reduced-thickness portion; and conducting a heat treatment under a temperature lower than a melting point of the base material, so that the cracked portion is refilled with a solidified mixed material, or the reduced-thickness portion is cladded, the following problems are raised. Namely, segregation of the melting point lowering agent is liable to occur, and the mixed material and the base material are bonded without completely melting the base material, so that there cannot be obtained a sufficient bonding strength at a boundary surface between the two materials. As a result, there cannot be obtained a fatigue strength or ductility corresponding to those of the base material.
Further, even in any of other conventional techniques, the oxidized films formed on the cracks caused during the operation of the gas turbine cannot be easily removed. Therefore, the removing operation had been performed in such a manner that the cracks including closed cracks each having a length exceeding an allowable range had been completely removed by grinding or gouging, thereby to perform the repairing operation. Accordingly, it requires much labor for grinding or gouging the cracked portion, thus resulting in high-cost situation.
Patent Abstracts of Japan
It is an object of the invention to provide a method of regenerating a gas-turbine stator vane, which is simply to be performed and results in a regenerated gas-turbine stator vane of a good quality.
In order to overcome and solve the problems that the amount of repairing work is increased as the operating temperature of the gas turbine is heightened, the heat affect, the residual stress and the amount of deformation are increased, and that it is difficult to correct the deformation even in the correction step, the inventors of the present invention had conducted much experiments and researches.
As a result, as one reason for increasing the heat affect and the residual stress in the gas turbine stator vane to be an object of the repairing operation, the inventors had paid attention to a point of that the amount of grinding or gouging requiring to remove the oxidized portion of the stator vane at which cracks had occurred.
Namely, when a periodic inspection or the like is conducted and a total length of the cracks formed to the stator vane is found to be longer than a chord length of the stator vane, such a point in time is recognized as a time for repairing the gas turbine stator vane. Then, all of the oxidized layers formed on the cracks having a non-allowable size and all of the cracks including the closed crack of which end portion is closed are ground to be completely removed. Thereafter, a repair welding is performed. Therefore, it takes much working hours to perform the grinding work due to a large amount of grinding. In addition, a welding amount for the repair is also greatly increased, thus resulting in a large amount of deformation after completion of the repairing operation.
The reason why the above problems are raised is as follows. Namely, although a thickness of the oxidized film formed on an inner surface of the crack occurred at the stator vane is small, it has been considered that a removing work by grinding all of the oxidized films together with all of the cracks is essentially required for completely repairing the stator vane.
In contrast, according to investigations conducted by the inventors of this invention, the following facts became clear. Namely, at a time of performing the heat treatment for the diffusion-brazing under a pressurized inert gas atmosphere, when a self-cleaning function of the brazing material i.e., a function of dissolving and removing the oxidized film from the base material is intensified, it became clear that the repairing operation by brazing could be effectively performed without grinding thin oxidized films formed on inner surface of the crack.
Further, the following fact also became clear. That is, in general, a residue of the melting point lowering components such as silicon (Si), boron (B) or the like contained in the brazing material becomes a factor of lowering a strength of the repaired portion. However, when the diffusion of the melting point lowering components to the base material is promoted, an amount of the melting point lowering components remained in the brazing material can be decreased, so that it becomes possible to remove the factor of lowering the strength of the brazing material.
Furthermore, the following fact also became clear. Namely, when the brazing material was steadily penetrated into a tip end portion of the crack formed in the base material, the diffusion of the melting point lowering agent into the base material became possible to be more surely promoted.
The aforementioned object of the present invention is achieved by realizing the above three functions, i.e., (i) the function of decreasing a thermal deformation caused by a local heat input during the repair-welding using the diffusion-brazing method, or decreasing a deformation caused by a residual stress during the welding work; (ii) the function of saving man-power by intensifying the self-cleaning function resulting in reduction of an amount of the base material to be ground; (iii) the function of improving the strength of a repairing portion in accordance with a method in which the repairing portion is sealed by a heat treatment for heating a material layer of which melting point is lowered, then under the sealed condition, the sealed portion is pressurized whereby the brazing material is penetrated and diffused into the crack.
The present invention provides a method of regenerating gas-turbine stator vane composed of base material and having cracks formed in the base material and oxidized layer formed at surface portion of the base material according to claim 1.
It is further preferable that when a total length of the cracks is longer than a chord length of the stator vane, the oxidized layer formed on entire surface of the stator vane is ground and the cracked portions are ground without completely grinding the cracked portion so that a part of the cracks remains, then an entire stator vane is covered with the equivalent material and the brazing material so as to completely repair the cracks.
The temperature for the heat treatment under the pressurized atmosphere is set to a range of 1150°C to 1260°C, and most preferably set to a range of 1150°C to 1210°C.
Furthermore, it is also preferable that the equivalent material has a composition containing 20-35 wt% of Cr, 5-60 wt% of Ni, 0.5-2 wt% of Fe, 5-10 wt% of W, 0.1-0.5 wt% of C, 0.005-2 wt% of B, and balance of Co, while the brazing material has a composition containing 10-40 wt% of Cr, 8.5-70 wt% of Ni, 0.5-2 wt% of Fe, 9 wt% or less (including 0%) of W, 0.001-0.6 wt% of C, 0.01-3.5 wt% of B, 1.0-11 wt% of Si, 2 wt% or less (not including 0%) of Mn, and balance of Co.
Still further, it is also preferable that the brazing material has a composition containing 10-40 wt% of Cr, 8.5-70 wt% of Ni, 0.5-2 wt% of Fe, 9 wt% or less (including 0%) of W, 0.001-0.6 wt% of C, 0.01-3.5 wt% of B, 1.0-11 wt% of Si, 2 wt% or less (not including 0%) of Mn, and balance of Co.
Furthermore, it is also preferable that the equivalent material has a composition containing 5-35 wt% of Cr, 5-75 wt% of Ni, 2 wt% or less (including 0%) of Fe, 12 wt% or less (not including 0%) of W, 0.6 wt% or less (not including 0%) of C, 1 wt% or less (including 0%) of B, 2 wt% or less (including 0%) of Hf, 6 wt% or less (not including 0%) of Ti, 3 wt% or less (not including 0%) of Nb, 5 wt% or less (including 0%) of Re, 5 wt% or less (including 0%) of Mo, 8 wt% or less (not including 0%) of Ta, 65 wt% or less (not including 0%) of Al, 0.7 wt% or less (including 0%) of Zr, and balance (5-65%)of Co, while said brazing material has a composition containing 10-40 wt% of Cr, 8.5-70 wt% of Ni, 0.5-2 wt% of Fe, 9 wt% or less (including 0%) of W, 0.001-0.6 wt% of C, 0.01-3.5 wt% of B, 1.0-11 wt% of Si, 2 wt% or less (not including 0%) of Mn, and balance of Co.
According to the present invention, the cracked portion can be completely repaired by using the diffusion-brazing treatment without requiring to completely grind and remove the cracks including closed cracks formed at surface of the gas turbine stator vane. At this time, when the heat treatment for the diffusion-brazing treatment is conducted under the pressurized inert gas atmosphere, it becomes possible to strengthen the self-cleaning function of dissolving the oxidized film thereby to reduce a thickness thereof, so that the repairing operation using the brazing material becomes possible without grinding the thin oxidized film. In addition, the diffusion of the melting point lowering agent into the base material can be promoted, so that it becomes possible to remove a factor of lowering the strength of the repaired portion. Further, the brazing material can be penetrated into a tip portion of the crack thereby to firmly bond the crack with the brazing material, thus capable of exhibiting the aforementioned three functions.
By constituting in this manner, there can be provided a method of regenerating gas-turbine stator vane, which can perform a regenerating treatment in which the gas-turbine stator vane formed with deteriorated portion or damages such as crack can be efficiently regenerated without requiring to completely grind and remove the cracks including the closed cracks, and without requiring an extra correction of deformation because the deformation can be small due to less heat affect and residual stress, thereby to provide a regenerated stator vane having a high quality.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.
Referring to the accompanying drawings, an embodiment of the present invention will be now explained.
As shown in
Namely, with respect to the closed cracks, a surface layer portion partially including the cracks is uniformly ground, while a deep portion of the cracks is remained. Under this state of the part of the cracks being remained, the heat treatment is conducted under the pressurized inert gas atmosphere. As a result, it becomes possible to strengthen the self-cleaning function and to fill the brazing material into the tip portions of the cracks. Notes, when a periodic inspection or the like is conducted and a total length of the cracks formed to the stator vane is found to be longer than a chord length of the stator vane, such stator vane shall be determined as an object to be subjected to this regenerating treatment.
Next, as shown in
In the present embodiment, a Co-based super alloy (trade name: FSX414) was used as the base material for constituting the stator vane. On the other hand, the equivalent material 3 having a composition containing 20-35 wt% of Cr, 5-60 wt% of Ni, 0.5-2 wt% of Fe, 5-10 wt% of W, 0.1-0.5 wt% of C, 0.005-2 wt% of B, and balance of Co, was applied to the equivalent material 3. In Table 1 shown hereunder, examples of the compositions of the base material 1, the equivalent material 3 and the brazing material 4 are indicated.
By the way, an equivalent material corresponding to the base materials such as, for example, In939 (trade name), GTD222 (trade name) or the like other than the material shown in Table 1 can be also adopted as a material for constituting the stator vane. For example, these materials have a composition containing 5-35 wt% of Cr, 5-75 wt% of Ni, 2 wt% or less (including 0%) of Fe, 12 wt% or less (not including 0%) of W, 0.6 wt% or less (not including 0%) of C, 1 wt% or less (including 0%) of B, 2 wt% or less (including 0%) of Hf, 6 wt% or less (not including 0%) of Ti, 3 wt% or less (not including 0%) of Nb, 5 wt% or less (including 0%) of Re, 5 wt% or less (including 0%) of Mo, 8 wt% or less (not including 0%) of Ta, 65 wt% or less (not including 0%) of Al, 0.7 wt% or less (including 0%) of Zr, and balance (5-65%)of Co.
Further, as the brazing material 4, the brazing material having a composition containing 10-40 wt% of Cr, 8.5-70 wt% of Ni, 0.5-2 wt% of Fe, 9 wt% or less (including 0%) of W, 0.001-0.6 wt% of C, 0.01-3.5 wt% of B, 1.0-11 wt% of Si, 2 wt% or less (not including 0%) of Mn, and balance of Co is adopted. One example of the composition of the brazing material is shown in above Table 1. That is, the brazing material 4 is a powder to which several percent of boron (B) and silicon (Si) are added so that a melting point of the material is lowered.
According to the present invention, the mixture of the equivalent material 3 and the brazing material 4 is filled into the ground portion 1 a so as to form a three-layered structure comprising: an outermost layer (upper side of the base material shown in
In another structure, it is preferable that an amount ratio of the brazing material is step-wisely or continuously increased in a range from the intermediate layer toward the outermost layer or the base-side layer.
After the repairing material is filled into the repairing portion, a heat treatment is conducted under a pressurized atmosphere composed of inert gas such as Ar gas or the like, so that the brazing material 4 is molten and a diffusion-brazing into portions of the cracks 2 is performed. For example, as shown in
Concretely, when a solution heat treatment and an aging heat treatment are conducted after completion of the heat treatment under the pressurized atmosphere, the repairing portion can be effectively repaired without causing a deformation during the repairing operation. In this regard, a pressure condition for the pressurizing treatment is set to 100 - 200 MPa. Further, the solution heat treatment and the aging heat treatment are performed at a temperature lower than a temperature at which the base material is partially molten, or performed at a temperature lower than a temperature at which a cell structure formed of eutectic carbide is collapsed. Concretely, the temperature for the solution heat treatment or the aging heat treatment is set to a range of 1100°C to 1300°C.
After the repairing portion is held in the furnace under the above conditions for about 30 minutes to several hours, the repairing portion is cooled to a room temperature so as to solidify the molten repairing material (S105), and then, the stator vane is taken out from the HIP furnace (S106). Subsequently, the stator vane is subjected to the solution heat treatment (S107) and the aging heat treatment (S108) in a vacuum furnace, thereby to complete the repairing operation.
Next, concrete examples of the present invention will be explained hereunder.
Table 2 listed hereunder shows a relationship between the pressure and the self-cleaning function when the pressing treatment is performed in the present embodiment.
As is clear from the results shown in above Table 2, according to the repaired portions of Examples 1 to 5 that were subjected to the heat treatment at a temperature of 1100°C or higher and at a pressure of 95 MPa or higher, each of the thickness of the oxidized films was 0.072 mm or less, although the sample of stator vane of Comparative Examples 1 that was not subjected to any treatment showed an oxidized layer having a large thickness of 0.8 mm.
In contrast, in cases of samples of Comparative Example 1 that was not treated, Comparative Example 2 that was treated at a pressure of 90 MPa, and Comparative Example 3 that was treated at a temperature of 900°C, each of the thickness of the oxidized films was 0.75 mm or more. Therefore, it was confirmed that the self-cleaning function could not be obtained in these Comparative Examples.
In view of the above facts, the effect of the self-cleaning function was remarkably recognized when the samples were repaired under the temperature condition of 1100°C or higher and at a pressure of 95 MPa or higher.
Next, Table 3 listed hereunder shows results of a tensile test conducted after the samples of Examples 6-8 according to the present invention and Comparative Examples of 4 and 5 were processed.
The above Table 3 shows results of the tensile test conducted at normal temperature with respect to the samples of Examples 6-8 that had been processed in accordance with a method comprising the steps of: cutting out a sample having a crack from a stator vane assembled in an actual gas turbine; grinding a surface of a cracked portion at a grinding amount of 0.5-1.5 mm; repairing a ground portion by brazing; conducting HIP treatment at a temperature of 1150°C and a pressure of 1000 MPa; conducting a solution heat treatment as an ordinary heat treatment at a temperature of 1150°C for two hours; and conducting an aging heat treatment at a temperature of 950°C for four hours.
Table 3 also shows the results of the tensile test with respect to the samples of Comparative Examples 4-5 that had been repaired in accordance with a conventional TIG (tungsten inert gas) weld-repairing method.
As is clear from the results shown in Table 3, in case of the samples of Examples 6-8, it was confirmed that strength properties equivalent to those of the base material could be obtained in regardless of the grinding amount and the residual amount of the cracks.
Table 4 shows results of measuring a grinding amount and a deformation amount with respect to the samples of the stator vanes of Examples 9-10 that had been processed in accordance with a method comprising the steps of: taking out a sample of stator vane having a cracks from a gas turbine actually-operated for 60000 hours; grinding a surface of a cracked portion at a grinding amount of 1-1.5 mm; and uniformly repairing the ground portion by brazing method under a pressurized atmosphere.
Table 4 also shows the results of Comparative Example 6 that had been repaired in accordance with a conventional method in which a cracked portion of the stator vane was completely ground and removed, and then TIG weld-repairing operation was performed to the ground portion. Each of the deformation amount of the samples was measured by a drop-checking method (deformation amount test).
As is clear from the results shown in Table 4, it was confirmed that the deformation amount of Comparative Example 6 was large to be 4.2 mm, whereas the deformation amounts of Examples 9, 10 were extremely small to be +0.2 to -0.3 mm. In these Examples 9,10, a penetrated crack was repaired so that the crack was blocked up by conducting only the brazing treatment. Therefore, a local heat input was not imparted at all to the base material of the stator vane, whereby the deformation of the stator vane was hardly observed even before and after the processing the stator vane.
Table 5 shows results of measuring: a degree of denseness of an intermediate layer in which a mixing ratio of the equivalent material and the brazing material was changed; and a degree of filling the repairing material into the cracked portion in the respective Examples 11-15 and Comparative Examples 7-11.
Table 5 also shows the results of the tensile test with respect to test pieces mainly sampled from each of the intermediate layers of the respective stator vanes.
(Mixing Ratio/Equvatent Material:Brazing Material)
(°C)
(MPa)
Layer
Layer 1)
Layer 2
Layer 3
Layer
(Voids)
The stator vanes of Examples 11-15 and Comparative Examples 7-11 were regenerated in accordance with the method comprising the steps of: grinding the oxidized layer and the cracks so that a part of the cracks remains thereby to form a ground portion; filling into the ground portion the repairing material consisting of the equivalent material and the brazing material, and having a mixing ratios shown in Table 5 thereby to form a filled portion comprising an outermost layer, at least one intermediate layer and a base-side layer; heat treating the filled portion under a temperature of 1210°C for 10 minutes so as to melt the brazing material and to seal off the repairing portion by a solidified brazing material so that the outermost layer can accept a pressing effect by HIP treatment; conducting the HIP treatment under the conditions shown in Table 5 thereby to perform a brazing treatment by diffusing the molten brazing material into the cracked portions; conducting a solution heat treatment at a temperature of 1150°C for four hours; and conducting an aging heat treatment at a temperature of 980°C for four hours. Then, the respective samples including the repaired portion were set so as to perform a structure observation and a mechanical test.
As a result, according to Examples 11-15 in which the mixing ratio of the brazing material was increased in the outermost layer and the base-side layer, while the amount ratio of the powder having a composition equivalent to that of the base material was increased in the intermediate layer, it was confirmed that all the degree of denseness, the degree of filling, strength, ductility (tensile elongation) were excellent in comparison with those of Comparative Examples 7-11 in which the mixing ratio of the brazing material was not increased in the outermost layer and the base-side layer, while the amount ratio of the powder having a composition equivalent to that of the base material was not increased in the intermediate layer.
Next, Table 6 listed hereunder shows results of structure-observations and tensile tests of the repaired portions in Examples 16-23 and Comparative Examples 12-13. Namely, Table 6 shows the results of evaluation for the respective structures comprising the base material and the brazed portion, and shows results of evaluation for the states of the repairing materials being filled into the crack, when the HIP conditions of temperature and pressure were changed as shown in Table 6.
(°C)
(MPa)
(MPa)
(%)
The repaired portions of Examples 16-23 and Comparative Examples 12-13 were processed in accordance with the method comprising the steps of: preparing a first sheet member having a composition containing 50% of the equivalent material and 50% of the brazing material; preparing a second sheet member having a composition containing 80% of the equivalent material and 20% of the brazing material; disposing the first sheet member as the outermost layer and the base-side layer (bottom) on the repairing portion; disposing the second sheet member as the intermediate layer between the outermost layer and the base-side layer; heat treating the three-layered structure under a temperature of 1210°C for 10 minutes so as to melt the brazing material; conducting the HIP treatment under the conditions shown in Table 6 thereby to perform a brazing treatment by diffusing the molten brazing material into the cracked portions. Thereafter, each of samples was subjected to a solution heat treatment at a temperature of 1150°C for four hours; and an aging heat treatment at a temperature of 980°C for four hours. Then, the respective samples including the repaired portion were set so as to perform the structure observation and the mechanical test.
As a result, as shown in table 6, according to Comparative Example 12 in which HIP treatment was performed at 1100°C or lower, the filling of the repairing material into the crack could not be sufficiently realized. In addition, the structure of the repaired portion could not reveal a uniform structure in which the brazing material and the equivalent material were completely alloyed, so that a sufficient strength and ductility (tensile elongation) could not be obtained.
In contrast, according to Examples 16, 17 in which HIP treatment was performed at 1100°C or higher, both the structure of the repaired portion and the degree of filling of the repairing material into the crack could be sufficiently improved, so that there could be obtained the good characteristics of both strength and ductility equivalent to the base material. Further, in case of Examples 18, 19 in which HIP treatment was performed at 1150°C or higher, the improvement was further advanced, so that the characteristics almost the same as those of the base material could be obtained.
In the repaired portion of Example 20 prior to HIP treatment, there was formed a cell structure composed of carbide which had been formed during a casting operation of the base material. However, in case of Example 20 in which HIP treatment was performed at 1210°C or higher, a part of carbide was dissolved to form a solid-solution, so that there was a sign of starting to collapse the cell structure.
Further, in case of Examples 21-23 in which HIP treatment was performed at 1260°C or higher, there was a sign of starting to partially collapse the cell structure, and a sign of lowering the characteristics of the repaired portion. In case of Comparative Example 13 in which HIP treatment was performed at 1300°C or higher, the collapse of the cell structure was clearly observed, so that remarkable lowering of the characteristics was recognized.
In
As is clear from the results shown in
That is, B and Si having an effect of lowering the melting point of the brazing material were dispersed and penetrated into the base material side after the regenerating treatment was conducted. As a result, a melting point of the boundary region between the brazed portion and the base material was heightened. Therefore, in a case where the repaired stator vane is reused, a heat resistant effect of the brazed portion is improved, and a strength of the stator vane operated at a high temperature is increased, so that a sufficient durability can be exhibited.
Namely, according to this embodiment, it was confirmed that a segregation of the elements contained in the brazing material, particularly the elements such as B, Si or the like having a function of lowering the melting point of the brazing material was greatly improved. Further, Cr content was also decreased at the repairing face as the same manner as B, Si contents.
As a result, according to this embodiment, the following excellent functions can be exhibited. Namely, at the time of completely repairing a defected stator vane by using a diffusion-brazing method, when the heat treatment for the diffusion-brazing method is conducted under a pressurized inert gas atmosphere, the self-cleaning function can be strengthened.
Further, it becomes possible to perform a brazing-repair for the defected stator vane without grinding a thin oxidized film formed on a surface of the crack. In addition, a diffusion of the melting point lowering agent into the base material is promoted, and a factor of lowering the strength can be removed. Furthermore, the brazing material can be penetrated into a tip portion of the crack.
The invention may be embodied in other specific forms without departing from the scope of the invention as defined in the appended claims. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.